The present invention relates to a low-pressure die-casting machine and method, and more particularly, to a die-casting machine and method for successively transporting molten metal from a melting pot to the casting port of a metal mold and filling the cavity of the metal mold with the molten metal. Hereinafter, introducing molten metal into a ladle so that the molten metal accumulates therein for transporting the molten metal from the melting pot to the metal mold will be referred to as "supply." Hereinafter, discharging molten metal from the ladle into a casting port of the metal mold will be referred to as "pouring."
Problems have been observed when automatically pouring a molten metal having a small mass, such as from 5 grams to several hundreds grams, into a casting port of a die-casting machine. That is, maintaining accuracy of the pouring amount and preventing the molten metal from cooling in the ladle have proven difficult. Several proposals have been made to overcome these problems.
For example, Japanese Patent No. 87747 discloses a piston/cylinder arrangement in which a piston is slidably disposed in a cylinder whose one end is formed with a molten metal intake/discharge port. The piston is slidingly moved in one direction while the cylinder is dipped in the molten metal accumulated in a melting pot., This generates negative pressure within the cylinder so the molten metal flows into the cylinder through the molten metal intake/discharge port. The piston is later slidingly moved in the opposite direction, whereupon the molten metal retained in the cylinder is discharged into a metal mold through the molten metal intake/discharge port.
Further, Japanese Patent Application Kokai No. SHO 50-13225 discloses a device having a support tube movable in the vertical direction and rotatable about its axis and a plurality of casting tubes radially extending from the support tube. An air intake passage and air chamber are formed in the support tube. The air chamber is in communication with the casting tubes through arm tubes. Generating negative pressure at the air intake passage applies negative pressure to the arm tubes through the air chamber whereupon molten metal accumulated in a pot is sucked into the casting tubes. While maintaining this state, the support tube is rotated to bring the casting tube into a predetermined position corresponding with a casting port of a metal mold.
Furthermore, Japanese Utility Model Application Kokai No. HEI-2-42751 describes a pouring device having a ladle for rotary style low pressure casting. The ladle has an intake/discharge port formed in the lower tip. A plug is movably provided in the interior of the ladle for closing and opening the intake/discharge port. The ladle is lowered into the molten metal until the intake/discharge port is dipped therein. The interior of the ladle is then decompressed via a pressurizing/decompressing device so that the molten metal is sucked into the interior of the ladle. After a predetermined amount of molten metal is supplied to the ladle, the plug seals the intake/discharge port. The ladle is then moved to a casting port of the metal mold and brought into sealing contact therewith. Before the plug is moved away from the discharge port, the pressurizing/decompressing device slightly decompresses the interior of the ladle so that when the plug opens the intake/discharge port, pouring is performed in a controlled manner. The interior of the ladle is then returned to atmospheric pressure to expedite the rate of pouring.
The above described conventional molten metal supplying devices have several disadvantages. First, in the Japanese Patent 87747 reference, the molten metal may enter into the sliding portion of the cylinder/piston mechanism. If molten metal solidifies there, subsequent molten metal cannot be supplied. This problem tends to worsen the smaller the amount of molten metal supplied. This is because smaller amounts of molten metal have smaller heat capacity so the temperature of smaller amounts rapidly decreases. Further, if the molten metal enters into the opposite side of the piston, the reciprocating mechanism there may also be damaged. Moreover, the suction of the molten metal by negative pressure may cause excessive turbulence in the molten metal in the cylinder. If the cylinder is elevated while the molten metal is turbulent, the amount of the molten metal within the cylinder cannot be accurately gauged, which in turn degrades accuracy in the molten metal supply. On the other hand, waiting for the molten metal to become stationary requires a relatively prolonged period.
According to Japanese Patent Application Kokai No. 50-13225, the plurality of casting tubes radially provided around the support tube requires intricate molten metal passages be formed within the support tube and the casting tubes. This complicated structure increases production cost of the device. Further, since the molten metal is sucked under negative pressure, the molten metal abruptly enters the casting tubes. This could cause the molten metal to enter into the molten metal passages in the arm tubes. If the molten metal adheres to the walls of the passage and solidifies there, a clog may form which prevents subsequent supply of molten metal. Further, the device has drawbacks similar to those of the above described patent reference since a vacuum suction system is also utilized.
The same problems can also occur in the pouring device described in Japanese Utility Model Application Publication (Kokai) No. HEI-2-42751 which also uses a pressurizing/decompressing device for supplying molten metal into the ladle. Also because the supply/discharge port is sealed and opened using a plug, impurities, for example aluminum, that solidify and accumulate around the exterior of the intake/discharge port can prevent the ladle from coming into sealing contact with the casting port, thereby obstructing pouring. Also if aluminum impurities deposit to the interior of the intake/discharge port or the exterior of the plug, the plug will not form a complete seal with the intake/discharge port so that molten metal will drip from the ladle during transport of the molten metal. The low pressure of the ladle interior at the time of pouring can cause air to enter therein, thereby generating voids in the casted product. Pouring can become impossible at certain levels of decompression. Because many trials must be performed to determine the most appropriate pressure, adjusting pressure is troublesome.